I generally find the white background of all the applications particular Word, Excell and web browser very tiring if you work long hours on big screen > 25 inches.
In the window that opens, click in the area that says "Window Text"
The Drop Box text should say "Window"
Now you can choose the foreground and background colors.
Once happy with your changes, "OK" your way out.
Caveat: Not all programs are going to pay attention to these new settings. I'd say most native Windows will, but others may not
Tip: You can change many window properties here; click the Drop Box for a long list of choices